There has recently been renewed interest in the therapeutic uses of medical cannabis and synthetic cannabinoids, such as Δ9-tetrahydrocannabinol (THC) [1], the active component of cannabis.

THC may be therapeutically beneficial in several major areas of medicine including control of acute and in particular chronic/neuropathic pain, nausea, anorexia, AIDS, glaucoma, asthma and in multiple sclerosis [Baker, D. et al, Nature 2000, 404, 84-87; Baker, D. et al, FASEB J. 2001, 15, 300-302; Schnelle, M. et al, Forsch. Komplementarmed. 1999, 6 Suppl 3, 28-36].
A number of cannabinoid ligands have been reported in the literature. Broadly speaking, cannabinoid ligands may be divided into three main groups consisting of (i) classical cannabinoids, such as (−)-Δ9-tetrahydrocannabinol, Δ9-THC [1] and CP55,940 [9]; (ii) endocannabinoids, such as anandamide [2] and 2-arachidonoyl glycerol [3]; and (iii) non-classical heterocyclic analogues typified by heterocycles such as WIN 55,212 [7] and the selective CB1 antagonist SR141716A [8] [Pertwee, R. G., Pharmacology & Therapeutics 1997, 74, 129-180]. Conformationaily restricted anandamide analogues have also been reported [Berglund, B. A. et al, Drug Design and Discovery 2000, 16, 281-294]. To date, however, the therapeutic usefulness of cannabinoid drugs has been limited by their undesirable psychoactive properties.

Cannabinoids are known to modulate nociceptive processing in models of acute, inflammatory and neuropathic pain [Pertwee, R. G., Prog. Neurobiol. 2001, 63, 569-611]. More specifically, research has centred on the role of cannabinoids in models of neuropathic hyperalgesia [Herzberg, U. et al, Neurosci. Lett. 1997, 221, 157-160] and inflammatory hyperalgesia [Richardson, J. D., Pain 1998, 75, 111-119; Jaggar, S. I. et al, Pain 1998, 76, 189-199; Calignano, A. et al, Nature 1998, 394, 277-281; Hanus, L. et al, Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 14228-14233]. It has also been suggested that cannabinoid receptor expression and the level of endogenous cannabinoids may change during inflammation and hyperalgesia [Pertwee, R. G., Prog. Neurobiol. 2001, 63, 569-611]. The cannabinoid signaling system is thought to involve two cloned cannabinoid receptors (CB1 and CB2), endocannabinoid ligands such as anandamide [2] and 2-arachidonoyl glycerol [3], and an endocannabinoid degradation system [Howlett, A. C. et al, International Union of Pharmacology XXVII, Pharmacol. Rev. 2002, 54, 161-202; Pertwee, R. G., Pharmacology of cannabinoid receptor ligands. Curr. Med. Chem. 1999, 6, 635-664].
One important function of the cannabinoid system is to act as a regulator of synaptic neurotransmitter release [Kreitzer, A. C. et al, Neuron 2001, 29, 717-727; Wilson, R. I. et al, Neuron 2001, 31, 453-462]. CB1 is expressed at high levels in the CNS, notably the globus pallidus, substantia nigra, cerebellum and hippocampus [Howlett, A. C., Neurobiol. Dis. 1998, 5, 405-416]. This is consistent with the known adverse effects of cannabis on balance and short-term memory processing [Howlett, A. C. et al, International Union of Pharmacology XXVII, Pharmacol. Rev. 2002, 54, 161-202]. CB2 is expressed by leucocytes and its modulation does not elicit psychoactive effects; moreover it does not represent a significant target for symptom management where the majority of effects are CB1 mediated.
Although many cannabinoid effects are centrally-mediated by receptors in the CNS [Howlett, A. C. et al, International Union of Pharmacology XXVII, Pharmacol. Rev. 2002, 54, 161-202], it is understood that peripheral CB receptors also play an important role, particularly in pain and in the gastrointestinal tract. For example, CB1 is also expressed in peripheral tissues, such as in dorsal root ganglia, peripheral nerves and neuromuscular terminals, thereby allowing neurotransmission to be regulated outside the CNS [Pertwee, R. G., Life Sci. 1999, 65, 597-605]. Accordingly, therapeutic activity in conditions such as those involving pain [Fox, A. et al, Pain 2001, 92, 91-100] or gut hypermotility, may be located in non-CNS sites. To date, however, research into the peripheral cannabinoid system has been hampered by the lack of pharmacological agents that selectively target peripheral receptors over those of the CNS.
In order to eliminate adverse psychoactive effects, it is desirable to exclude cannabinoid agonists from the CNS. There are two established methods for CNS exclusion of small molecule agents. Firstly, one method involves excluding substances from the CNS by carefully controlling their physicochemical properties so as to prevent them crossing the blood brain barrier (BBB). The BBB is formed by brain endothelial cells, with tight intercellular junctions and little fenestration [Tamai, I. et al, J. Pharm. Sci. 2000, 89, 1371-1388]. Consequently, substances must enter the brain either by passive diffusion across plasma membranes or by active transport mechanisms. The BBB thus forms an effective barrier to many peripherally circulating substances.
An alternative method of excluding compounds from the brain is to incorporate structural features which enable them to be actively pumped across the BBB. One such example is the opioid agonist loperamide; although lipophilic, loperamide contains structural features recognized by the p-glycoprotein transporter (MDR1) that allow it to be actively pumped across the blood brain barrier. [Wandel, C. et al, Anesthesiology 2002, 96, 913-920; Seelig, A. et al, Eur. J. Pharm. Sci. 2000, 12, 31-40].
The present invention seeks to provide new cannabinoid receptor modulators. More particularly, the invention seeks to provide cannabinoid receptor modulators that alleviate and/or eliminate some of the disadvantages commonly associated with prior art modulators, for example undesirable psychoactive side effects. More specifically, though not exclusively, the invention seeks to provide modulators that selectively target peripheral cannabinoid receptors.